This invention relates to a system for producing sulfur, and more particularly to a modular system for reducing sulfur dioxide by contacting same with coal at elevated temperatures.
Hydrocarbon fuels which are normally burned in industrial installations, such as coal and oil-fired power stations, contain sulfur which, under normal circumstances, is converted to sulfur dioxide in the combination process. Although in older installations the sulfur dioxide was vented to atmosphere with the other effluent gases from the process, recent air pollution control requirements have placed great emphasis on removing the sulfur dioxide from the gases before the latter are expelled into atmosphere. One successful technique in this respect is to remove the sulfur dioxide from polluted gas streams by adsorption on carbon to form sulfuric acid, followed by a regeneration of the adsorbent to produce a stream of high sulfur dioxide concentration.
In order to form a non-polluting by-product that can be accumulated in a useful form, various systems have also been proposed for converting the sulfur dioxide obtained in the foregoing manner to elemental sulfur. However, many of these latter systems require the use of natural gas or some other relatively expensive reducing agent. In this context, U.S. patent application Ser. No. 489,337, filed July 17, 1974 by Peter Steiner and assigned to the assignee of the present invention discloses a process in which a gas containing sulfur dioxide is contacted with granular coal to produce sulfur. This has the advantage of utilizing crushed coal, which is the least expensive reducing agent, and is thus very attractive from a cost standpoint.
However, according to this process, a single reactor vessel is provided through which the coal is continuously passed and partially combusted in the presence of the sulfur dioxide containing gas to reduce the gas to sulfur. However, the reactor vessel, being a high temperature gas-solid contact device, is subject to heat and mass transfer limitations as to its maximum practical size. As a result, in relatively large systems, these limitations would necessitate the design and construction of a multitude of single reactors. This, of course, is relatively inefficient, especially from a materials standpoint.